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FFmpeg multithreading methods==============================================FFmpeg provides two methods for multithreading codecs.Slice threading decodes multiple parts of a frame at the same time, usingAVCodecContext execute() and execute2().Frame threading decodes multiple frames at the same time.It accepts N future frames and delays decoded pictures by N-1 frames.The later frames are decoded in separate threads while the user isdisplaying the current one.Restrictions on clients==============================================Slice threading -* The client's draw_horiz_band() must be thread-safe according to the comment in avcodec.h.Frame threading -* Restrictions with slice threading also apply.* For best performance, the client should set thread_safe_callbacks if it provides a thread-safe get_buffer() callback.* There is one frame of delay added for every thread beyond the first one. Clients must be able to handle this; the pkt_dts and pkt_pts fields in AVFrame will work as usual.Restrictions on codec implementations==============================================Slice threading - None except that there must be something worth executing in parallel.Frame threading -* Codecs can only accept entire pictures per packet.* Codecs similar to ffv1, whose streams don't reset across frames, will not work because their bitstreams cannot be decoded in parallel.* The contents of buffers must not be read before ff_thread_await_progress() has been called on them. reget_buffer() and buffer age optimizations no longer work.* The contents of buffers must not be written to after ff_thread_report_progress() has been called on them. This includes draw_edges().Porting codecs to frame threading==============================================Find all context variables that are needed by the next frame. Move allcode changing them, as well as code calling get_buffer(), up to beforethe decode process starts. Call ff_thread_finish_setup() afterwards. Ifsome code can't be moved, have update_thread_context() run it in the nextthread.If the codec allocates writable tables in its init(), add an init_thread_copy()which re-allocates them for other threads.Add CODEC_CAP_FRAME_THREADS to the codec capabilities. There will be very littlespeed gain at this point but it should work.Call ff_thread_report_progress() after some part of the current picture has decoded.A good place to put this is where draw_horiz_band() is called - add this if it isn'tcalled anywhere, as it's useful too and the implementation is trivial when you'redoing this. Note that draw_edges() needs to be called before reporting progress.Before accessing a reference frame or its MVs, call ff_thread_await_progress().